1. Field of the Invention
The invention relates to a semiconductor sensor for measuring the flow rate of a flowing fluid, e.g. a liquid or a gas.
2. Description of the Prior Art
In general, the principle of a hot-wire anemometer for measuring the flow of a fluid has been known for a long time. More recently, proposals have been made to implement semiconductor sensors based on this principle.
For example, U.S. Pat. No. 4,680,963 describes a semiconductor sensor integrating a heating resistance and associated thermometers on a single silicon substrate. The center of the face of the substrate which is subjected to the fluid flow is provided with a heating resistance, and its periphery is provided with thermometers which are isolated from the heating resistance by oxidized porous silicon. Given that the thermal conductivity of oxidized porous silicon is up to 100 times less than that of silicon, this prevents thermal short circuits occurring in the substrate between the heating resistance and the thermometers.
The sensor described above nevertheless suffers from drawbacks insofar as the heating resistance and the thermometers require a protective layer in order to avoid the implanted circuits for heating and for measuring temperature suffering problems of abrasion, of corrosion, and of ion contamination. Unfortunately, making such a layer gives rise to several difficulties. Firstly it must have thermal expansion characteristics that are substantially identical to those of the substrate to avoid measurement fluctuations because of stresses due to temperature variations. In addition, it must have good thermal conductivity so as to avoid opposing the transfer of heat between the heating resistance and the fluid; with this latter condition nevertheless presenting a drawback insofar as it gives rise to a new thermal bridge in the protective layer between the heating resistance and the thermometers.
An object of the invention is to remedy these drawbacks by proposing a solution that is effective and cheap.
The proposed flow rate sensor comprises a substrate of silicon having a first face provided with a heating element disposed in a first region of said substrate, together with at least one thermometer component disposed in a second region of the substrate, said first and second regions being insulated from each other by a third region of said substrate formed at least in part by oxidized porous silicon; according to the invention, the sensor is adapted to receive the fluid flow over the second face of said substrate, with said first and second regions forming respective thermal short circuits between the first and second faces of said substrate.
Advantageously, the third region is constituted by at least one insulating channel surrounding the first region. In a particular embodiment, the first region is substantially cylindrical with the third region being annular.
In a variant, the third region is constituted by a succession of SiO.sub.2 channels separated by Si.
A cap is fixed on the first face of said substrate so as to stiffen the substrate, said cap possessing a cavity formed facing the first region and at least a portion of the third region.
Advantageously, the cavity is formed facing the first region and a portion of the third region. Advantageously, the interface between the second and third regions lies outside the cavity. When the first region is substantially cylindrical and the third region is constituted by at least one insulating channel surrounding the first region, the cavity is circular in outline.
Once the substrate has been consolidated and stiffened in this way, it may be machined to adapt it to the flow of a fluid over its second face.
In a first variant embodiment, in order to receive the fluid flow, the sensor includes a recess formed in the second face of the substrate, with all three regions opening out at the same level in the bottom of the recess.
In a particular embodiment, the recess is in the form of a cross centered on the first region and having one of its branches corresponding to the flow axis of the fluid.
In another variant embodiment, in order to receive the fluid flow, the second face of the substrate is plane, with the second face of the substrate being worked so that the thickness of the substrate is no greater than the thickness of the insulating third region.
Regardless of whether the second face is worked by forming a recess therein such that the bottom of the recess and the first face of the substrate form a membrane, or by machining the entire second face to reduce the thickness of the substrate, the working of the second face serves to provide thermal insulation laterally between the first and second regions.
Advantageously, the cap is made of silicon; the bond between the cap and the first face of the substrate may be constituted by electrostatic sealing via a fine layer of Pyrex. In a variant, the bond between the cap and the first face of the substrate may be made via a layer of siloxane deposited by centrifuging and sealed by pressure at a temperature lying in the range 200.degree. C. to 450.degree. C. The space between the cap and the first face may be filled with an inert gas.
The first face of the substrate may advantageously be provided with at least one thermometer component disposed in the first region.
The silicon substrate is preferably of the P-type and the heating element and the thermometer components are implanted directly in the first face of said substrate.